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Fri, 18 Sep 2026 21:57:11 -0700 (PDT) Content-Type: text/plain; charset=UTF-8 Date: Sat, 19 Sep 2026 04:57:10 +0000 Message-Id: Cc: "Andrii Nakryiko" , "Daniel Borkmann" , "Eduard Zingerman" , Subject: Re: [PATCH bpf-next 07/20] bpf: Refuse exception cleanup shapes bpf_throw() cannot dispatch From: "Alexei Starovoitov" To: "Yonghong Song" , In-Reply-To: <20260917055721.3930090-1-yonghong.song@linux.dev> References: <20260917055645.3926444-1-yonghong.song@linux.dev> <20260917055721.3930090-1-yonghong.song@linux.dev> X-Mailer: mkdraft (claude review draft; edit before sending) Content-Transfer-Encoding: 8bit Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Wed, Sep 16, 2026 at 10:57 PM Yonghong Song wrote: > diff --git a/kernel/bpf/exception.c b/kernel/bpf/exception.c > index dfe9c2a9ce7d..b2bf831242b8 100644 > --- a/kernel/bpf/exception.c > +++ b/kernel/bpf/exception.c [...] > +static enum cleanup_insn_kind cleanup_classify(struct bpf_verifier_env *env, u32 i, > + int *next, int *target) > +{ > + struct bpf_insn *insn = &env->prog->insnsi[i]; > + u8 class = BPF_CLASS(insn->code); > + > + *next = i + 1; > + *target = -1; > + > + if (insn->code == (BPF_LD | BPF_IMM | BPF_DW)) { > + *next = i + 2; > + return CLEANUP_INSN_PLAIN; > + } > + if (class != BPF_JMP && class != BPF_JMP32) > + return CLEANUP_INSN_PLAIN; robot voice: LD_ABS/LD_IND is not plain. bpf_check_cfg() treats it as an abnormal return and gen_ld_abs() later patches in a real "r0 = 0; exit" for the failed load. With this classification a pad in a tc prog that keeps skb in r6 and does an ld_abs passes t he "reaches no exit" check, cleanup_check_pad_insn() doesn't look at it, and check_ld_abs() only wants no locks/refs held at that point. At run time a failed load runs the subprog epilogue on the walker's stack: callee regs get popped from arch_bpf_run_cleanup_pad()'s frame, leave/ret with rbp == the unwinding frame goes straight back into the BPF caller with garbage in rbx/r13-r15, and the rest of bpf_throw() is abandoned. Treat it like BPF_EXIT here or refuse it in a pad body like tail call.